Integrated amp Marantz PM5004 bias and upgrade

[fjadouce100]

1. Due to degraded sound in my Marantz PM5004 , I checked the bias VRs and found that the bias were 2.1V and 3.2V LR channels. I have readjusted the bias according to the chinese manual i.e 5mV but still with the recommended setting I was not satisfied cause the sound was still dull and lacked lower end. I have slightly increased to 7 mV and immediately there was a noticeable improvement. I would like to know how much more increase is possible to get the best sound and at the same time avoid damage to the power transistors 2SA1649 2SC4467. With 7mV the heat sink is just slightly warm.
2. I would like to upgrade the power capacitors Koshin 6800uF x2. What would you recommend?
3. Is it possible to increase just slightly the input signal in the preamp so as to get a little more output from the speakers without any distortion? If so how to proceed? I am just a hobbyist and have no special equipment .. no oscilloscope etc to check the signal waveform.
4. Lastly what other upgrades can be done to improve the sound?

 

[nicozero]

Hi fjadouce100,

get a try to replace these caps:

C215/216, C235/236 & C286/287 -> 10µF/50V -> DC blocking caps -> nichicon muse
C288/289 220pF/100V -> preamp LPF -> silver mica or styroflex
C304/305 ->100µF/25V -> feedback loop -> elna silmic II

 

[fjadouce100]

Thank u. Will order these components soon.
Anyone can give advice regarding increasing bias? to what extent?

 

[nicozero]

I don't know about bias and about Koshin caps quality but some like Mundorf, Nichicon or Pana are good ones. It will depend of dimensions, space and your budget!